A preemptive strike against Iran would violate both U.S. and international law.

Israeli Prime Minister Benjamin Netanyahu's visit to Washington has provoked a broad debate over the military and political wisdom of an attack on Iran. But so far, there has been little attention to the legal issues involved, which are crucial. American support for a preemptive strike would be a violation of both international law and the U.S. Constitution.

Article II of the Constitution requires the president to "take care that the laws be faithfully executed," and Article VI says that treaties are part of the "supreme law of the land." Since the Senate overwhelmingly ratified the United Nations Charter as a treaty in 1945, the president is constitutionally required to abide by Article 51 of the charter. This provision allows states to use military force in self-defense only when responding to an "armed attack." Preemptive attacks are another matter.

In 1981, the United States joined in the U.N. Security Council's unanimous condemnation ofIsrael'spreemptive assault on an Iraqi nuclear reactor. British Prime Minister Margaret Thatcher put it bluntly: "Armed attack in such circumstances cannot be justified. It represents a grave breach of international law."

In standing with the Security Council to condemn the Israeli raid, the Reagan administration was embracing a tradition of U.S. statesmanship that began with Secretary of State Daniel Webster. In 1837, the British were trying to suppress a revolt in eastern Canada. Because U.S. militias were assisting the uprising, the British launched a night raid into New York state, burning a U.S. ship, the SS Caroline, and sending it over Niagara Falls.

After lengthy negotiations, Webster gained British consent to a treaty that prohibited such preemptive strikes. The two sides agreed in 1842 that a cross-border strike was legitimate only if there was a "necessity of self-defense, instant, overwhelming, leaving no choice of means, and no moment for deliberation." This Anglo-American formula remains a part of international law today.

The United States was also the central player at the decisive moment for self-defense in the 20th century: the judgment at Nuremberg. We remember these trials for their condemnation of genocide. But this was not their central focus. The main charge was that the Nazis had waged aggressive war — and this required the Allies to endorse the limited doctrine of self-defense enshrined in traditional law.

Even when the United States felt itself to be directly threatened during the 1962 Cuban missile crisis, President Kennedy did not invoke the right of preemptive self-defense. Although the risk of mass destruction was high, the president's legal arguments were carefully constrained: When intercepting Soviet missiles on the high seas, Kennedy relied on the regional peacekeeping provisions of the U.N. Charter.

A departure from this restrictive approach came only recently, during the run-up to the war in Iraq, when the George W. Bush administration pointed to Saddam Hussein's purported looming nuclear threat to American cities as justification for the U.S.-led invasion. The tragic outcome of this adventure only emphasizes the wisdom of Webster's insistence that the "necessity of self-defense" be "instant" and "overwhelming."

Today, we are at a crucial legal turning point. If President Obama supports Netanyahu's preemptive strike, he will transform Bush's Iraq aberration into the founding precedent of a new era of international law. He should instead reaffirm Reagan's position in 1981 and return the presidency to its traditional commitments to international law abroad and constitutional fidelity at home.

The wrong choice would have profound consequences. We are moving into a multipolar world, where the United States and its allies will have diminished power to secure the peace. This is not the time to unleash an open-ended doctrine of preemptive self-defense that will permit other nations to avoid Security Council approval for the aggressive use of military force.

This moment of decision comes at an awkward time, given election-year politics. But it is the president's job to govern according to law while pursuing the long-run interests of the United States.

Bruce Ackerman is a professor of law and political science at Yale and the author, most recently, of "The Decline and Fall of the American Republic."